In recent years, due to demands for multimedia services, or the like, the amount of information transmitted via a network has been increasing; therefore, there is a need for large-volume transmission systems. Furthermore, as one of the technologies for increasing a transmission capacity, wireless communication systems that use Orthogonal Frequency Division Multiplexing (OFDM) have been put into practical use. The wireless communication systems that use the OFDM are also called, for example, 4th-generation mobile communication systems or 4G systems. The standards of the 4G systems include, for example, Long Term Evolution (LTE) or LTE-Advanced (LTE-A). In the wireless communication system that uses the OFDM, signals in various multilevel modulation methods depending on usage are superimposed on subcarriers that are orthogonal to each other, and the signals are transmitted.
In the above-described wireless communication system, a power amplifier is used on the transmitting side for transmitting signals. The input/output characteristics of the power amplifier exhibit linearity if the output is small, and they exhibit nonlinearity due to saturation if the output is large. For example, if the power amplifier is operated at a high efficiency in the vicinity of the saturation range, the input/output characteristics of the power amplifier exhibit nonlinearity. Due to the nonlinearity, intermodulation distortion (IMD) occurs. The IMD is defined by amplitude modulation-amplitude modulation (AM-AM) type distortion (i.e., “amplitude distortion”) and amplitude modulation-phase modulation (AM-PM) type distortion (i.e., “phase distortion”). If IMD occurs, an unnecessary distortion component leaks into an adjacent channel, and a distortion occurs in and out of the band of the signal. As a result, it causes interference.
Here, the techniques for compensating IMD of a power amplifier include Digital Pre-Distortion (DPD). The DPD is a process to superimpose a distortion compensation coefficient, which serves as a distortion component with the characteristics opposite to the nonlinear distortion characteristics of the power amplifier, on the signal before it is input to a power amplifier. The distortion compensation coefficient is referred to from the lookup table (LUT) on the basis of the instantaneous power of the signal that is input to the power amplifier.    [Non Patent Literature 1] L. Ding et al., “A Robust Digital Baseband Predistorter Constructed Using Memory Polynomials,” IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 52, NO. 1, pp. 159-165, January 2004.    [Non Patent Literature 2] L. Ding et al., “A MEMORY POLYNOMIAL PREDISTORTER IMPLEMENTED USING TMS320C67XX,” Proceedings of Texas Instruments Developer Conference, 2004.    [Non Patent Literature 3] O. Hammi et al., “Digital Sub-band Filtering Predistorter Architecture for Wireless Transmitters,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 5, pp. 1643-1652, MAY 2005.    [Non Patent Literature 4] Hsin-Hung Chen et al., “Joint Polynomial and Look-Up-Table Predistortion Power Amplifier Linearization,” IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-II: EXPRESS BRIEFS, VOL. 53, NO. 8, 612-616, AUGUST 2006.
However, not only in-band and out-of-band IMD based on the instantaneous power, but also in-band IMD, which occurs due to memory effect, is present in the IMD of a power amplifier. This is the distortion that occurs due to that the output of the power amplifier depends on not only the value of the signal (instantaneous power) currently input to the power amplifier, but also the value (log) of the signal previously input to the power amplifier. Therefore, there is a case where, in order to also compensate distortion that occurs due to the memory effect, a memory polynomial is applied to the DPD (see Non Patent Literatures 1 to 4). However, if a memory polynomial is applied to the DPD, the amount of calculations for distortion compensation is enormous, and the power consumption, which is consumed during signal processing such as calculations, is increased.